So you should start by visiting AMO and looking to see if your add-on needs any work done at all.

Note: You should still test your add-on on Firefox 9, even if it's been automatically upgraded. There are edge cases that may not be automatically detected.

Once you've confirmed that you need to make changes, come on back to this page and read on.

Bootstrapped add-ons can remove delayed-load scripts

If your add-on uses nsIChromeFrameMessageManager.loadFrameScript() with the delayed-load flag set, the script gets loaded into every frame created from that point on. This is great, except that until Firefox 9, there was no way to stop loading the script, so it would keep happening even after your add-on was shut down.

The nsIGlobalHistory3 interface has been removed. Its functionality was of limited (if any) use to add-ons, so this shouldn't affect anyone.

Several specialized channels' properties attributes have been merged into the base nsIChannel interface. This shouldn't affect compatibility at all, since those interfaces inherit from nsIChannel anyway.

Preference changes

The geo.wifi.* preferences no longer have default values, although they're honored if they exist. If your code reads these without handling the case where they don't exist, you need to update your code to handle the exception that gets thrown when they're not present.

XPConnect changes

nodePrincipal and baseURIObject have been moved from nsDOMClassInfo to XrayWrapper. This shouldn't affect many add-ons, since it would only be an issue if they try to access these properties on DOM Node objects from unprivileged script that have requested XPConnect privileges using enablePrivilege().

DOM changes

The long-obsolete method Navigator.taintEnabled() has been removed. This hasn't done anything useful in a very long time, but was often used in browser detection scripts, since it was Netscape-specific. Calling this method throws an exception starting in Firefox 9.

Event handlers are now implemented as standard IDL interfaces. For most cases, this won't affect you, but there are exceptions.

Other changes that may affect binary compatibility

These changes are notable in that they may affect binary XPCOM components. These will need rebuilding anyway, since that's required for every major release of Firefox, but could introduce compile-time errors, so they're worth noting in particular.

Theme changes

The pending attribute has been added to the tab element. If this attribute is present, the tab is in the process of being restored by the session store service. You can use that to style the tab during the restore process. It's worth noting that if the user has turned on the "Don't load tabs until selected" preference, the pending attribute is set on tabs until they get loaded.

Similarly, tabs also now have an unread attribute; this property, if present, indicates that the tab has changed since the last time it was the active tab. You can use this to style tabs differently when they have changed since the last time the user looked at them. This is also present on tabs that have not yet been looked at during the current session.